Plant lipids have a variety of industrial and nutritional uses and are central to plant membrane function and climatic adaptation. These lipids represent a vast array of chemical structures, and these structures determine the physiological and industrial properties of the lipid. Many of these structures result either directly or indirectly from metabolic processes that alter the degree of saturation of the lipid.
Plant lipids find their major use as edible oils in the form of triacylglycerols. The specific performance and health attributes of edible oils are determined largely by their fatty acid composition. Most vegetable oils derived from commercial plant varieties are composed primarily of palmitic (16:0), stearic (18:0), oleic (18:1), linoleic (18:2) and linolenic (18:3) acids. Palmitic and stearic acids are, respectively, 16- and 18-carbon-long, saturated fatty acids. Oleic, linoleic, and linolenic acids are 18-carbon-long, unsaturated fatty acids containing one, two, and three double bonds, respectively. Oleic acid is referred to as a mono-unsaturated fatty acid, while linoleic and linolenic acids are referred to as poly-unsaturated fatty acids. The relative amounts of saturated and unsaturated fatty acids in commonly used, edible vegetable oils are summarized below (Table 1):
TABLE 1 ______________________________________ Percentages of Saturated and Unsaturated Fatty Acids in the Oils of Selected Oil Crops Saturated Mono-unsaturated Poly-unsaturated ______________________________________ Canola 6% 58% 36% Soybean 15% 24% 61% Corn 13% 25% 62% Peanut 18% 48% 34% Safflower 9% 13% 78% Sunflower 9% 41% 51% Cotton 30% 19% 51% ______________________________________
Many recent research efforts have examined the role that saturated and unsaturated fatty acids play in reducing the risk of coronary heart disease. In the past, it was believed that mono-unsaturates, in contrast to saturates and poly-unsaturates, had no effect on serum cholesterol and coronary heart disease risk. Several recent human clinical studies suggest that diets high in mono-unsaturated fat and low in saturated fat may reduce the "bad" (low-density lipoprotein) cholesterol while maintaining the "good" (high-density lipoprotein) cholesterol (Mattson et al., Journal of Lipid Research (1985) 26:194-202). Soybean oil is high in saturated fatty acids when compared to other sources of vegetable oil and contains a low proportion of oleic acid, relative to the total fatty acid content of the soybean seed. These characteristics do not meet important health needs as defined by the American Heart Association.
A soybean oil low in total saturates and polyunsaturates and high in monounsaturate would provide significant health benefits to the United States population, as well as, economic benefit to oil processors.
Oil biosynthesis in plants has been fairly well-studied see Harwood (1989) in Critical Reviews in Plant Sciences, Vol. 8 (1):1-43!. The biosynthesis of palmitic, stearic and oleic acids occur in the plastids by the interplay of three key enzymes of the "ACP track": palmitoyl-ACP elongase, stearoyl-ACP desaturase and the acyl-ACP thioesterases.
Of these three enzyme types, the acyl-ACP thioesterases function to remove the acyl chain from the carrier protein (ACP) and thus from the metabolic pathway. The oleoy-ACP thioesterase catalyzes the hydrolysis of oleoyl-ACP thioesters at high rates and at much lower rates the hydrolysis of palmitoyl-ACP and stearoyl-ACP. This multiple activity leads to substrate competition between enzymes and it is the competition of this acyl-ACP thioesterase and palmitoyl-ACP elongase for the same substrate and of acyl-ACP thioesterase and stearoyl-ACP desaturase for the same substrate that leads to a portion of the production of the palmitic and stearic acids found in the triacylglyceride of vegetable oils.
Once removed from the ACP track fatty acids are exported to the cytoplasm and there used to synthesize acyl-coenzyme A. These acyl-CoA's are the acyl donors for at least three different glycerol acylating enzymes (glycerol-3-P acyltransferase, 1-acyl-glycerol-3-P acyltransferase and diacylglycerol acyltransferase) which incorporate the acyl moieties into triacylglycerides during oil biosynthesis.
These acyltransferases show a strong, but not absolute, preference for incorporating saturated fatty acids at positions 1 and 3 and monounsaturated fatty acid at position 2 of the triglyceride. Thus, altering the fatty acid composition of the acyl pool will drive by mass action a corresponding change in the fatty acid composition of the oil.
Based on the above discussion, one approach to altering the levels of palmitic, stearic and oleic acids in vegetable oils is by altering their levels in the cytoplasmic acyl-CoA pool used for oil biosynthesis.
In previous work (WO 9211373) Applicant has demonstrated that oleoyl-ACP thioesterase may be modulated using cloned cDNA encoding the soybean enzyme. Oleoyl-ACP thioesterase cDNA was used to form chimeric genes for the transformation of soybean plant cells resulting in the anti-sense inhibition of acyl-ACP thioesterase in the plant seed.
Applicant has now discovered an entirely new plant thioesterase with activity on a C16 substrate that is also useful for the regulation of the acyl coenzyme A pool. Applicant has isolated nucleic acid fragments that encode soybean and canola palmitoyl-ACP thioesterases that are useful in modifying fatty acid composition in oil-producing species by genetic transformation. Thus, transfer of the nucleic acid fragments of the invention or a part thereof that encodes a functional enzyme, along with suitable regulatory sequences that direct the transcription of their mRNA, into a living cell will result in the production or over-production of palmitoyl-ACP thioesterases and will result in increased levels of saturated fatty acids in cellular lipids, including triacylglycerols.
Transfer of the nucleic acid fragments of the invention or a part thereof, along with suitable regulatory sequences that direct the transcription of their anti-sense RNA, into plants will result in the inhibition of expression of the endogenous palmitoyl-ACP thioesterase that is substantially homologous with the transferred nucleic acid fragment and will result in decreased levels of saturated fatty acids in cellular lipids, including triacylglycerols.
Transfer of the nucleic acid fragments of the invention or a part thereof, along with suitable regulatory sequences that direct the transcription of their mRNA, into plants may result in inhibition by cosuppression of the expression of the endogenous palmitoyl-ACP thioesterase gene that is substantially homologous with the transferred nucleic acid fragment and may result in decreased levels of unsaturated fatty acids in cellular lipids, including triacylglycerols.